Now we will begin the left thumb gusset
Row 1: Knit 4, M1 (make one), PM (place a marker), knit to end of row.
Above picture shows how I M1 (make 1). I lift the stitch with the left needle and knit into the back of it so as not to leave a hole.
Knitting in the front and back of the stitch to increase does not work - it does not leave a smooth increase line and does not look quite as nice.
Above picture is the first row knit to the end with the marker in position.
You can see I have 5 sts at the right because I M1 and 29 sts on the left.
Row 2: Purl to marker, slip marker and m1.
To make one on the purl side, lift the stitch as above and
come in from behind to purl this stitch. We don't knit it because we do not want any purl bumps on the right side. It's a wee bit tricky but there are only a few more rows of this type of increase.
Rows 3-11: Repeat last 2 rows 4 times more.
Keep increasing every row until 44 (50) total stitches
You should end up with 15 sts on the right side of the marker and 29 sts on the left side.
Let me know if you get stuck anywhere and I'll see if I can better explain.
Or if I miscalculated the row counts.
Mostly it is the number of stitches at the end that matter.